1. Field of the Invention
The present invention relates to a reflective liquid crystal display device and a liquid crystal display device operable both in a reflection mode and a transmission mode, and which are used for office automation equipment such as wordprocessors and personal computers, mobile information devices such as hand-held computers, and VTRs integrated with a camera and having a liquid crystal monitor. The present invention also relates to a method for producing such liquid crystal display devices. In this specification, a liquid crystal display device will be referred to as an xe2x80x9cLCD devicexe2x80x9d. A liquid crystal display device operable both in a reflection mode and a transmission mode will be referred to as a xe2x80x9ctransmission- and reflection-type LCD devicexe2x80x9d.
2. Description of the Related Art
LCD devices do not emit light themselves unlike CRTs (cathode ray tubes) and EL (electroluminescence) devices. Accordingly, transmissive LCD devices equipped with a backlight on a rear surface thereof are used.
The backlight usually consumes 50% or more of the total power consumption of the LCD device. Some mobile information devices which are often used outdoors or constantly carried by the user include a reflective LCD device which includes a reflective plate and performs display using only the ambient light.
Reflective LCD devices include TN (twisted nematic) mode devices and STN (super twisted nematic) mode devices which use a polarizer and are in a wide use as transmissive LCD devices today, as well as phase change (PC) guest-host mode devices which have been actively developed recently. The PC guest-host mode devices do not use a polarizer and thus realize brighter display. Such a device is disclosed in, for example, Japanese Laid-Open Publication No. 4-75022 corresponding to U.S. Pat. No. 5,220,444 and Japanese Laid-Open Publication No. 9-133930.
However, the PC guest-host mode LCD devices perform display using optical absorption by dyes in a liquid crystal layer including liquid crystal molecules and the dyes dispersed therein. Accordingly, the phase transition guest-host mode LCD devices provide significantly lower quality than the TN devices and the STN devices using a polarizer.
In LCD devices including the liquid crystal molecules aligned in parallel or in a twisted manner, the liquid crystal molecules at the center and in the vicinity of the liquid crystal layer tilt vertically to surfaces of substrates. However, the liquid crystal molecules in the vicinity of alignment layers do not tilt vertically to the surfaces of the substrates. Accordingly, the birefringence of the liquid crystal layer cannot be 0. Therefore, in the case where the LCD device operates in a display mode for performing black display when a voltage is applied, satisfactory black display is not performed due to the remaining birefringence. Thus, sufficient contrast ratio is not obtained.
The TN mode and STN mode devices do not provide sufficiently high quality display in terms of brightness and contrast. Accordingly, further improvement in the brightness and the contrast is demanded.
Reflective LCD devices are disadvantageous in that the intensity of the reflected light used for display is lowered when the ambient light is dark. By contrast, transmissive LCD devices are disadvantageous in that the visibility is lowered when the ambient light is very bright, for example, outdoors on a fine day.
According to one aspect of the invention, a liquid crystal display device includes a first substrate; a second substrate; a liquid crystal layer interposed between the first substrate and the second substrate; a first polarizer provided on a surface of the first substrate which is on the opposite side to the liquid crystal layer; a second polarizer provided on a surface of the second substrate which is on the opposite side to the liquid crystal layer; a first phase compensation element provided between the first polarizer and the liquid crystal layer; and a second phase compensation element provided between the second polarizer and the liquid crystal layer. A plurality of pixel areas are provided for display. The first substrate includes at least one transmissive electrode, and the second substrate includes a reflective electrode region and a transmissive electrode region in correspondence with each of the plurality of pixel areas.
In one embodiment of the invention, each of the plurality of pixel areas has a reflective region for performing display using reflected light and a transmissive region for performing display using transmitted light, and the reflective electrode region defines the reflective region and the transmissive electrode region defines the transmissive region.
In one embodiment of the invention, the liquid crystal layer has a retardation of zero when a molecular axis of liquid crystal molecules in the liquid crystal layer is substantially vertical with respect to the surface of the first and second substrates, and the first phase compensation element and the second phase compensation element each have a retardation which fulfills xcex/4 condition.
In one embodiment of the invention, the liquid crystal layer has a retardation of xcex1 when a molecular axis of liquid crystal molecules in the liquid crystal layer is almost vertical with respect to the surface of the first and second substrates, and the first phase compensation element has a retardation which fulfills xcex/4-xcex1 condition.
In one embodiment of the invention, the liquid crystal layer has a retardation of xcex1 when a molecular axis of liquid crystal molecules in the liquid crystal layer is almost vertical with respect to the surface of the first and second substrates, the first phase compensation element has a retardation which fulfills xcex/4-xcex1 condition, and the second phase compensation element has a retardation which fulfills xcex/4-(xcex2-xcex1) condition.
In one embodiment of the invention, the first phase compensation element and the second phase compensation element are each formed of a xcex/4 wave plate, a transmission axis of the first polarizer and the first phase compensation element make an angle of about 45 degrees, and a transmission axis of the second polarizer and the second phase compensation element make an angle of about 45 degrees.
In one embodiment of the invention, the second phase compensation element is formed of a xcex/4 wave plate, and a slower optic axis of the second phase compensation element matches one of a longer axis or a shorter axis of elliptically polarized light transmitted through the liquid crystal layer and incident on the second phase compensation element so as to convert the elliptically polarized light into linearly polarized light, and a transmission axis of the second polarizer is perpendicular to a polarizing axis of the linearly polarized light.
According to another aspect of the invention, a liquid crystal display device includes a first substrate including a transmissive electrode; a second substrate including a reflective electrode; a liquid crystal layer interposed between the first substrate and the second substrate and including liquid crystal molecules which exhibit negative dielectric anisotropy and are aligned substantially vertically to surfaces of the first substrate and the second substrate when no voltage is applied; a polarizer provided on a surface of the first substrate which is opposite to the liquid crystal layer; and a xcex/4 wave plate provided between the polarizer and the liquid crystal layer. A slower axis of the xcex/4 wave plate and a transmission axis of the polarizer make an angle of about 45 degrees.
In one embodiment of the invention, the liquid crystal display device further includes a phase compensation element between the reflection electrode and the polarizer.
According to still another aspect of the invention, a liquid crystal display device includes a first substrate; a second substrate; a liquid crystal layer interposed between the first substrate and the second substrate and including liquid crystal molecules which exhibit negative dielectric anisotropy and are aligned substantially vertically to surfaces of the first substrate and the second substrate when no voltage is applied; a first polarizer provided on a surface of the first substrate which is on the opposite side to the liquid crystal layer; a second polarizer provided on a surface of the second substrate which is on the opposite side to the liquid crystal layer; a first xcex/4 wave plate provided between the first polarizer and the liquid crystal layer; and a second xcex/4 wave plate provided between the second polarizer and the liquid crystal layer. A plurality of pixel areas are provided for display. The first substrate includes at least one transmissive electrode, and the second substrate includes a reflective electrode region and a transmissive electrode region in correspondence with each of the plurality of pixel areas. Slower axes of the first xcex/4 wave plate and the second xcex/4 wave plate are in an identical direction and make an angle of about 45 degrees with each of transmission axes of the first polarizer and the second polarizer.
In one embodiment of the invention, each of the plurality of pixel areas has a reflective region for performing display using reflected light and a transmissive region for performing display using transmitted light, and the reflective electrode region defines the reflective region and the transmissive electrode region defines the transmissive region.
In one embodiment of the invention, the liquid crystal display device further includes at least one phase compensation element between the first polarizer and the second polarizer.
In one embodiment of the invention, the liquid crystal layer further includes a chiral dopant.
In one embodiment of the invention, the liquid crystal layer has an approximately 90 degree twisted orientation.
In one embodiment of the invention, the first polarizer and the second polarizer have transmission axes perpendicular to each other, and the first phase compensation element and the second phase compensation element have slower axes perpendicular to each other.
In one embodiment of the invention, the first phase compensation element converts linearly polarized light from the first polarizer into circularly polarized light, and the second phase compensation element converts linearly polarized light from the second polarizer into circularly polarized light, the liquid crystal display device further including a third phase compensation element provided between the first polarizer and the liquid crystal layer for compensating for wavelength dependency of refractive index anisotropy of the first phase compensation element.
In one embodiment of the invention, the third phase compensation element is a xcex/2 wave plate, and when a transmission axis of the first polarizer and a slower axis of the third phase compensation element make an angle of xcex31, the transmission axis of the first polarizer and a slower axis of the first phase compensation element make an angle of 2xcex31+45 degrees.
In one embodiment of the invention, the liquid crystal display device further includes a fourth phase compensation element provided between the second polarizer and the liquid crystal layer for compensating for wavelength dependency of refractive index anisotropy of the second phase compensation element.
In one embodiment of the invention, the fourth phase compensation element is a xcex/2 wave plate, and when a transmission axis of the second polarizer and a slower axis of the fourth phase compensation element make an angle of xcex32, the transmission axis of the second polarizer and a slower axis of the second phase compensation element make an angle of 2xcex32+45 degrees.
In one embodiment of the invention, the transmission axis of the first polarizer is perpendicular to the transmission axis of the second polarizer, a slower axis of the first phase compensation element is perpendicular to the slower axis of the second phase compensation element, and a slower axis of the third phase compensation element is perpendicular to the slower axis of the fourth phase compensation element.
According to still another aspect of the invention, a liquid crystal display device includes a first substrate; a second substrate; and a liquid crystal layer interposed between the first substrate and the second substrate. A plurality of pixel areas are provided for display, each of the plurality of pixel areas having a reflective region for performing display using reflected light and a transmissive region for performing display using transmitted light. The first substrate includes a counter electrode in the vicinity of the liquid crystal layer. The second substrate includes, in the vicinity of the liquid crystal layer, a plurality of gate lines, a plurality of source lines perpendicular to the plurality of gate lines, a plurality of switching elements provided in the vicinity of intersections of the plurality of gate lines and the plurality of source lines, a first conductive layer having a high light transmission efficiency, and a second conductive layer having a high light reflection efficiency, the first conductive layer and the second conductive layer being connected to each of the switching elements, connected to each other, and being provided in each of the pixel areas.
In one embodiment of the invention, the liquid crystal display device further includes an insulating layer between the first conductive layer and the second conductive layer.
In one embodiment of the invention, the second substrate further includes a third conductive layer, and the first conductive layer and the second conductive layer are connected to each other through the third conductive layer.
In one embodiment of the invention, one of the first conductive layer, the second conductive layer and the third conductive layer is formed of a material identical with one of materials forming the plurality of gate electrodes or the plurality of source electrodes.
In one embodiment of the invention, the insulating layer has a wave-like surface below the second conductive layer.
According to still another aspect of the invention, a method for producing a liquid crystal display device is provided. The liquid crystal display device includes a first substrate; a second substrate; and a liquid crystal layer interposed between the first substrate and the second substrate. A plurality of pixel areas are provided for display, each of the plurality of pixel areas having a reflective region for performing display using reflected light and a transmissive region for performing display using transmitted light. The first substrate includes a counter electrode in the vicinity of the liquid crystal layer. The second substrate includes, in the vicinity of the liquid crystal layer, a plurality of gate lines, a plurality of source lines perpendicular to the plurality of gate lines, a plurality of switching elements provided in the vicinity of intersections of the plurality of gate lines and the plurality of source lines, a first conductive layer having a high light transmission efficiency, a second conductive layer having a high light reflection efficiency, the first conductive layer and the second conductive layer being connected to each of the switching elements, connected to each other, and being provided in each of the pixel areas, and an insulating layer provided between the first conductive layer and the second conductive layer. The method includes the steps of forming the first conductive layer on a plate; forming the insulating layer at least on the first conductive layer; forming the second conductive layer on the insulating layer; and partially removing the second conductive layer formed on the first conductive layer.
In one embodiment of the invention, the method further includes the steps of forming a third conductive layer on a connection area, on at least the first conductive layer, for connecting the first conductive layer and the second conductive layer so as to connect the first conductive layer and the second conductive layer to each other through the third conductive layer; forming the insulating layer; and partially removing the insulating layer at least on the connection area for connecting the first conductive layer and the second conductive layer.
In one embodiment of the invention, the step of partially removing the insulating layer includes the step of removing the insulating layer on an area of the first conductive layer.
Thus, the invention described herein makes possible the advantages of providing a reflection-type LCD device and a transmission- and reflection-type LCD device providing satisfactory display with a sufficiently high contrast, and a method for producing the same.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.